Method for shaping ceramic articles



Dec. 13, 1955 M. SKUNDA 2,725,433

METHOD FOR SHAPING CERAMIC ARTICLES Filed Sept. 18, 1951 2 Sheets-Sheetl Jnventor (Ittornegs Dec. 13, 1955 M. SKUNDA METHOD FOR SHAPING CERAMICARTICLES 2 Sheets-Sheet 2 Filed Sept. 18 1951 aw M 9 (Ittornegs UnitedStates Patent METHOD FOR SHAPING CERAMIC ARTICLES Michael Skunda,Davison, Mich., assignor to General Motors Corporation, Detroit, Mich.,a corporation of Delaware Application September 18, 1951, Serial No.247,111

6 Claims. (Cl. -156) The present invention relates to ceramic articlesand particularly to a method for shaping such ceramic articles as sparkplug insulators by altering the contour of aceramic blank having acomposition including some softenable material such as a thermoplasticresin.

To provide for better insulating properties, it has been found desirableto provide annular rings and grooves on the surfaces of spark pluginsulators, thus allowing more surface distance between the spark pluglead connection and the grounded spark plug shell. To accomplish aninsulator with this increased complexity of contour, various methods andapparatus for manufacture have been suggested. One of these methods isto mold the insulator from a composition of ceramic material and binderas disclosed in U. S. Patent 2,446,872. Another is to mold the insulatorto finished contour at only some portions and then to provide additionalmaterial to the points where annular surface rings are desired, andremold to a finished product. Both of these methods, however, aredisadvantageous in that in either case the molding process results inflash at the mold parting line. Such a flash line is particularlyundesirable because in some cases it prevents the spark plug gasketsfrom sealing properly. These molding processes are also quite expensive,requiring precision tooling and yielding low product output. v

Another method for manufacturing spark plug insulators of complexcontour is to mold a blank of a composition containing a ceramicmaterial and resin binder, mount the blank for rotation along itslongitudinal axis, heat the material to a plastic condition and thenrotatably engage the surface of the blank with a series of rolls tocause annular rings and grooves at the desired places. This method isdisadvantageous in that it can be used only to produce less complicatedshapes and it is not readily adaptable to producing rings of differentcross-sectional areas on the same insulator.

The insulator could also be made from a blank of suitable material by agrinding operation.

complicated shapes. There is also considerable waste of insulatormaterial, thus making the process costly.

Another possible method is to mount an insulator blank for rotation onits longitudinal axis and engage the surface of the rotated blank with asurface cutting tool to trim away circumferential portions until aninsulator of the desired contour is obtained. Due, however, to theinherent abrasiveness of the material, tool life is extremely short,thus making the process costly. This cold turning method is alsodisadvantageous in that frictional heat builds up in the insulatormaterial and causes deformation.

It is therefore one of the objects of this invention to eliminate thedifliculties of the above mentioned methods by providing an efiicientand low cost method for producing spark plug insulators. I

Another object of this invention is to provide an eflicient method forproducing ceramic articles having complex contours. I

However, this; method, too, is adaptable only to the manufacture of'lessAnother object of this invention is to provide an eiiicient method foraltering the contour of previously formed ceramic articles.

Another object of this invention is to provide an efiicient process forremoving surface portions of ceramic articles.

Other objects and advantages will be seen more clearly from thefollowing description of my invention and more particularly from theappended claims.

In accordance with my invention, a blank made of a ceramic material andcontaining a thermoplastic binder is rotated on its axis while itssurface is engaged by any suitable cutting tool which is heated to abovethe softening temperature of the binder.

In the drawings:

Fig. 1 shows a side view of a ceramic material blank mounted for axialrotation.

Fig. 2 shows a top view of a surface skiving tool provided with aheating element and suitably mounted in cutting engagement with therotated blank. v

Figs. 3 and 4 show top views of progressive steps in the contouring ofthe article as successive portions of the skiving tool engage successiveportions along the length of the blank.

Fig. 5 shows a side view of a rotatably mounted ceramic spark pluginsulator after the skiving operation has been completed.

Fig. 6 is a view of the apparatus along the lines 66 of Fig. 5 and showsa cross-sectional view of the mounted insulator, the skiving tool andits heating and mounting means.

Fig. 7 shows a modification of the skiving tool and skiving tool heatingmeans.

In Fig. 1, a blank 1, made of a suitable thermoplastic bound ceramicmaterial is shown mounted for rotation. The particular blank shown has aperfectly cylindrical shape; however, any suitable shaped blank willsuffice, it being necessary only that suflicient material be provided toallow for the cross-sectional area requiredof thefinished article. Theblank can be formed in any suitable manner known in the art, i. e.molding, the exact process or apparatus used in this blank forming stepbeing no. The composition used to.

part of the present invention. make the blank can be any suitableceramic material together with any of a variety of thermoplastic binderswell known in the art. Compositions particularly adapted to themanufacture of spark plug insulators by my process and apparatus aredescribed in U. S. Patent 2,446,872. The blank 1 is mounted for rotationon rotatable shafts 2 which are in frictional engagement at 3 with theblank 1 through grommets 4, these grommets being made of any suitablefrictional material such as rubber. In the particular modificationshown, the blank is provided With a longitudinal bore 5 into which theends of the shafts'2 can be appropriately designed to fit; however, sucha structure is not, of course, essential. The shafts 2 are in drivingengagement with some driving means located in the casing 6.

and not shown, it being understood that either one or both of the shafts2 can be driven by suitable driving The entire assembly is supported onbase memmeans.

ber 7'.

In Fig. 2 the ceramic blank 1 is shownsupported by the shafts 2 andfrictionally engaged for rotation by grommets 4. Skiving tool 8 providedwith a cylindrical shank portion 9 is held adjustably positioned inclamp 10 by the screw 11, the screw 11 allowing for angular. adjustmentof the tool 8. A heating element 12 sur-Q rounds the shank portion 9 ofthe tool 8, the heat from this heating element being conducted throughthe shank 9 and the tool 8 to the tool edge 13. The-heating element 12is adapted to deliver tothe tooledge 13 sufiicient heat to maintain itabove the softening-"tern perature of the particular binder used inblank 1. The edge of the tool 13 is contoured so that it will give thedesired shape to the ceramic article. In the preferred embodiment of theprocess and apparatus, as shown by the accompanying drawings, the tool 8is set at an angle to the axis of rotation of the blank so that only oneportion of the tool edge 13 engages the surface of the blank 1 at anyone time. Tool edge 13 can be of any suitable material, tungsten carbidefor example, proving very satisfactory.

The edge 13 of the tool 8 engages the surface of the rotating blank 1and shaves or chips away portions of the surface of the blank accordingto the contour with which the tool edge is provided. In the preferredembodiment shown on Figs. 2, 3 and 4, as the heated skiving tool 8 ismoved across the blank 1, continuously successive portions of the tooledge 13 move into cutting engagement with continuously successivecircumferential portions along the length of the blank 1.

Fig. 2 shows the process at a stage where portion 14 of the tool edge 13is in cutting engagement with the blank 1. Fig. 3 shows a later stage inthe process, portion 15 of the tool edge 13 at this time being incutting engagement with the surface of the blank 1 as the tool is causedto move across the blank. Fig. 4 shows a still later stage in theprocess, at this time portion 16 of the tool edge 13 being in cuttingengagement with the blank 1. After portion 16 of the tool edge 13 hascompleted its skiving of the corresponding portion 17 of the ceramicarticle, the contouring operation is comp ete.

In accordance with my invention, therefore, a ceramic article having acomplex contour is simply and economically produced. An example of anarticle such as is produced by my invention is best shown in Fig. 5wherein a spark plug insulator 18 provided by the contouring operationwith circumferential rings and grooves 19, and tapered portions 20 and21, is shown still mounted in the apparatus.

While in the preferred embodiment of my invention, as shown in theaccompanying drawings and as described above, the contouring operationis performed with a skiving tool, it is to be understood that othertypes of forming tools may of course be used. The tool can be positionedto engage the blank either radially or tangentially and it may be set sothat the cutting edge is parallel with the work centerline rather thanat an angle. When, for example, a very small circumferential area of theblank is to be contoured, it is sometimes desirable to use this lattermodification wherein the tool edge is positioned parallel to the workcenterline.

After the contouring operation is complete the article can be fired toremove the binder and sinter the ceramic material into a coherent mass.

In Figs. 2 to 6 the tool heating means shown consists of an electricalheating element 12 surrounding shank 9 and in heat conductingrelationship with the tool edge through shank 9 and the body of the tool8. This heating apparatus is best indicated in Fig. 6 which shows across-sectional view of the apparatus. Fig. 7 shows a modified tool andtool heating system wherein the heating element 22, having electricleads 23, is located within a recess 24 provided on the tool 8' adjacentthe cutting edge 13. Fig. 7 also shows a thermocouple 25 located nearthe tool edge for close temperature control. Such a thermocouple can, ofcourse, also be used with the apparatus shown in Figs. 2 to 6 for closertemperature control. Controlled heating of the tool edge 13 can beaccomplished by either heating apparatus. However, I have found that forcloser control of the tool temperature, the apparatus as shown in Fig.7, wherein the heating element is closer to the cutting edge, isdesirable.

As has been previously stated, the temperature of the tool edge 13 mustbe maintained above the softening temperature of the binder used inmaking the blank.

Since one of the great advantages to my process is that the chips fromthe blank can be used again to make other blanks and thus materiallyreduce costs of production, in order to use my process mostadvantageously it is desirable that the tool temperature not exceed thetemperature at which the binder begins to char. When using a binder suchas is described in U. S. Patent 2,446,872 and which consists generallyof a mixture of ethyl cellulose, shellac and a suitable plasticizer suchas N-butyl stearate, I have found that a tool temperature of between 350F. and 650 F. is suitable for my process. When using such a binder and atool temperature of 350 F., the cutting action is smooth and the chip isglossy, pliable and continuous. At 600 F. the chip is continuous and soplastic that it gives the appearance of a strip being extruded from anozzle. Higher tool temperatures, i. e. 700 F. and above, can often beadvantageously employed depending on the charring characteristics of thebinder used. The cutting time required will of course depend on thespeed with which the tool is moved into or across the work. The mostadvantageous tool speed which can be used depends on the type of binderand the tool temperature employed and the rate of work rotation.

It is thought that the hot tool plasticizes the binder and greatlyreduces abrasiveness. The reduction of abrasiveness may be due to thebinder acting as a lubricant for the cutting tool.

Whatever may be the cause, I have found that with my process frictionalheat is minimized, thus dispensing with the problem of blank deformationwhich is due to a general softening of the binder throughout the blank.Also, a long tool life is obtained, thus resulting in low productioncosts. Ceramic articles with a complex contour, such as spark pluginsulators with circumferential rings and grooves, can be efficientlyproduced with great savings in time and cost.

While my invention has been described in conjunction with certainspecific examples, it is to be understood that the scope of my inventionis not to be limited thereby except as defined in the appended claims.

I claim:

1. A process for manufacturing spark plug insulators comprising thesteps of forming a mixture of ceramic material and heat-softening resinbinder, forming an elongated blank from said mixture, rotating saidblank on its longitudinal axis, heating the surface of the rotated blankto above the softening temperature of the resin binder, skiving portionsof said heated surface to shape said blank into an insulator of desiredcontour, and firing said insulator to remove the binder and sinter saidinsulator into a coherent body.

2. In a process for manufacturing a spark plug insulator the steps offorming a mixture of ceramic ma terial and a thermoplastic binder,forming said mixture into an elongated blank, rotating said blank alongits longitudinal axis, heating a cutting tool to above the softeningtemperature of said thermoplastic binder, and engaging the surface ofthe rotated blank with said cutting tool to remove portions of saidsurface.

3. In a process for manufacturing ceramic articles, the step of forminga mixture of ceramic and thermoplastic materials, shaping said mixtureinto a blank, heating an edged tool to above the softening temperatureof the thermoplastic material and engaging the surface of said blankwith said edged tool to remove portions of said surface.

4. A process for manufacturing ceramic articles including the steps offorming a mixture of ceramic and thermoplastic materials, shaping saidmixture into a blank, heating the surface of said blank to above thesoftening temperature of the thermoplastic material and removingportions of said heated surface to shape said blank into an article ofdesired contour.

5. In a process for manufacturing ceramic articles the 5 6 steps offorming a mixture of ceramic and thermoplastic References Cited in thefile of this patent materials, shapingsaid mixture into a blank,rotating said UNITED STATES PATENTS blank on its longitudinal axis,heatlng a cutting tool to above the softening temperature of thethermoplastic ma- 168'801 Stevens 1875 terial, and subjectingcontinuously successive portions of 716343 Locke 1902 the surface ofsaid rotated blank to continuously suc- 5 914,816 Dqnton 1909 cessiveportions of said cutting tool while maintaining said 9 9 9 Splller 1911cutting tool heated to above softening temperature of 1240575 JefierySept 1917 the thermoplastic materiaL Mccool p 51 6. In a process formanufacturing ceramic articles, the 1465157 Buckley 1923 step of forminga mixture of ceramic and thermoplastic 1O 1491079 chfa'mplon 1924materials, shaping said mixture into a blank, heating an 1755308 Smith1930 edged tool to above the softening temperature of the 2464'718Potter et 1949 thermoplastic material, engaging the surface of saidblank 2561050 Chamm et July 1951 with said edged tool to remove portionsof said surface 2580092 Herbert et 1951 and thereby shape said blankinto a body of desired 15 2,632,223 Jordan 1953 contour and thereafterfiring said body to remove said FOREIGN PATENTS thermoplastic materialand sinter said ceramic material. 278 181 Germany Sept. 22, 1914

1. A PROCESS FOR MANUFACTURING SPARK PLUG INSULATORS COMPRISING THESTEPS OF FORMING A MIXTURE OF CERAMIC MATERIAL AND HEAT-SOFTENING RESINBINDER, FORMING AN ELONGATED BLANK FROM SAID MIXTURE, ROTATING SAIDBLANK ON ITS LONGITUDINAL AXIS, HEATING THE SURFACE OF THE ROTATED BLANKTO ABOVE THE SOFTENING TEMPERATURE OF THE RESIN BINDER, SKIVING PORTIONSOF SAID HEATED SURFACE TO SHAPE SAID BLANK INTO AN INSULATOR OF DESIREDCONTOUR, AND FIRING SAID INSULATOR TO REMOVE THE BINDER AND SINTER SAIDINSULATOR INTO A COHERENT BODY.